Various techniques for casting molten metals and metal-matrix composites have been developed. Gravity casting, permanent mold casting, die casting, investment mold casting and squeeze casting commonly are exploited. However, pressure infiltration casting offers advantages over these methods. Besides overcoming the non-wettability of molten metal with a reinforcement, i.e., a preform, and the ability to rapidly prototype components prior to large scale production, pressure infiltration casting can produce near-absolute net-shape cast parts with low to negligible porosity. As a result, pressure infiltration castings are used in automotive, truck, heavy construction equipment and outboard motor applications. Pressure infiltration castings also may be used in aerospace and sports applications.
Pressure infiltration casting generally is a process where a pressure differential is used to move molten infiltrant into a mold cavity to produce a conventional monolithic casting, i.e., an unreinforced casting, having the shape of the mold cavity. Pressure infiltration casting also includes moving a molten infiltrant into a mold cavity containing a preform. A preform typically is another metal or ceramic, usually of a particular shape and size such as a fiber. A reinforced casting, e.g., a metal-matrix composite, results from infiltration of a preform.
Pressure infiltration casting processes typically evacuate a mold cavity before addition of molten infiltrant to reduce or eliminate porosity of the finished product due to trapped air. Using the proper techniques, pressure infiltration casting can produce net shape reinforced composites or conventional castings with dimensional tolerances of .+-.0.0002 inches with a surface finish of 4 microinches (about 0.1 .mu.m), i.e., a surface with a mirror-like finish.
The overall pressure infiltration casting process generally involves the steps of (1) heating a mold vessel containing a mold; (2) heating an infiltrant to a molten state; (3) evacuating the heated mold vessel; (4) adding the molten infiltrant to the evacuated heated mold vessel if not initially present in the mold vessel; (5) applying pressure to the molten infiltrant to move it into a mold cavity; and (6) solidifying the molten infiltrant to form a finished cast product. Certain of the above steps may be conducted simultaneously and in the same vessel. For example, the mold vessel and the infiltrant often are combined and heated in the same chamber of an apparatus, as are the steps of pressurizing and cooling often conducted in the same chamber, usually different from the heating chamber.
Heating the mold vessel, mold and infiltrant usually requires the greatest amount of time in the overall casting process. Infiltration of the mold cavity with the molten infiltrant typically is the fastest step, while solidification of the molten infiltrant in the mold takes longer than infiltration but less time than heating the mold vessel and infiltrant. Accordingly, the throughput of finished products, i.e., the number of parts cast per unit time, may be increased by shortening the length of time for an individual step in the overall process or by strategically segregating steps so certain tasks may be performed simultaneously.
Early pressure casting publications and patents generally disclose processes that use a one chamber apparatus to perform the whole casting process, i.e., heating, evacuating, adding infiltrant, pressurizing and cooling. See, U.S. Pat. No. 3,547,180 to Cochran, and U.S. Pat. No. 3,913,657 to Banker et al.; and DE 3603 310 A1 to Zapfe. State-of-the-art publications and patents generally disclose processes that use multi-chamber apparatus where typically the steps of heating and evacuating are separated from the steps of pressurizing and cooling. See, , U.S. Pat. No. 4,832,105 to Nagan et al., and U.S. Pat. No. 5,335,711 to Paine; and DE 3220 744 A1 to Reuter et al. and GB 2,195,277 A to Doriath et al. However, state-of-the-art processes typically heat and evacuate a mold vessel and infiltrant in the same chamber.
In the aforementioned processes, the one chamber or multi-chamber apparatus is in use during the full casting cycle thereby occupying the entire apparatus for every step of the process. Since the entire apparatus is in use even during the slowest steps of heating and cooling, expensive vacuum and pressure equipment and chambers are used for only a short period of time. Thus, state-of-the-art pressure infiltration casting processes, even using multi-chamber apparatus, have a limited throughput because of the heating, and to a smaller degree cooling, steps.
It had been discovered that the steps of heating and evacuating may be conducted in a vessel separate from pressuring and cooling, however, these methods typically require the use of a vent tube. See, U.S. Pat. Nos. 5,322,109 and 5,553,658 to Cornie, which are herein incorporated by reference in their entirety.
Additionally, state-of-the-art pressure infiltration casting solidification methods generally involve using heat sinks, a chill zone or chill plate. S, e.g., U.S. Pat. No. 3,770,047 to Kirkpatrick et al.; U.S. Pat. Nos. 5,111,870 and 5,111,871 to Cook; and U.S. Pat. No. 5,275,227 to Staub. A chill plate often is made of metal in the shape of a pedestal which is brought into contact with a heated mold vessel after pressure has driven the molten infiltrant into the mold cavities. The chill plate also may have active means for facilitating the heat transfer process such as fluid flowing through the interior of the chill plate or through coiled pipes. Since cooling tends to be the second longest step in the pressure infiltration casting process, state-of-the-art solidification techniques also limit the overall throughput of the pressure infiltration casting process.
Accordingly, there exists a need for improved methods for pressure infiltration casting which economically produce with increased throughput high quality cast parts. In addition, there exists a need for improved apparatus for conducting high throughput pressure infiltration casting.